X-ray reflectometry (XRR) is a well-known technique for measuring the thickness, density and surface quality of thin film layers deposited on a substrate. Such reflectometers typically operate by irradiating a sample with a beam of X-rays at grazing incidence, i.e., at a small angle relative to the surface of the sample, in the vicinity of the total external reflection angle of the sample material. Measurement of X-ray intensity reflected from the sample as a function of angle gives a pattern of interference fringes, which is analyzed to determine the properties of the film layers responsible for creating the fringe pattern.
A method for analyzing XRR data to determine film thickness is described, for example, in U.S. Pat. No. 5,740,226, to Komiya et al., whose disclosure is incorporated herein by reference. After measuring X-ray reflectance as a function of angle, an average reflectance curve is fitted to the fringe spectrum. The average curve is based on a formula that expresses attenuation, background and surface roughness of the film. The fitted average reflectance curve is then used in extracting the oscillatory component of the fringe spectrum. This component is Fourier transformed to find the film thickness.
U.S. Pat. No. 5,619,548, to Koppel, whose disclosure is incorporated herein by reference, describes an X-ray thickness gauge based on reflectometric measurement. A curved, reflective X-ray monochromator is used to focus X-rays onto the surface of a sample. A position-sensitive detector, such as a photodiode detector array, senses the X-rays reflected from the surface and produces an intensity signal as a function of reflection angle. The angle-dependent signal is analyzed to determine properties of the structure of a thin film layer on the sample, including thickness, density and surface roughness.
U.S. Pat. No. 5,923,720, to Barton et al., whose disclosure is incorporated herein by reference, also describes an X-ray spectrometer based on a curved crystal monochromator. The monochromator has the shape of a tapered logarithmic spiral, which is described as achieving a finer focal spot on a sample surface than prior art monochromators. X-rays reflected or diffracted from the sample surface are received by a position-sensitive detector.
U.S. Pat. Nos. 6,512,814 and 6,639,968, to Yokhin et al., whose disclosures are incorporated herein by reference, describe an X-ray reflectometry system that includes a dynamic shutter, which is adjustably positionable to intercept the X-rays incident on the sample. This shutter, along with other features of the system, permits detection of XRR fringe patterns with high dynamic range. These patents also disclose improved methods for analysis of the XRR fringe pattern in order to determine thin film properties, including density, thickness and surface roughness. The high dynamic range enables the system to determine these properties accurately not only for the upper thin film layer, but also for one or more underlying layers on the surface of the sample.
XRR may also be used in situ, within a deposition furnace, to inspect thin film layers in production on a semiconductor wafer, as described, for example, by Hayashi et al., in U.S. Patent Application Publication US 2001/0043668 A1, whose disclosure is incorporated herein by reference. The furnace is provided with X-ray incidence and extraction windows in its side walls. The substrate upon which the thin film has been deposited is irradiated through the incidence window, and the X-rays reflected from the substrate are sensed through the X-ray extraction window.
X-ray diffractometry (XRD) is a well-known technique for studying the crystalline structure of matter. In XRD, a sample is irradiated by a monochromatic X-ray beam, and the locations and intensities of the diffraction peaks are measured. The characteristic scattering angles and the scattered intensity depend on the lattice planes of the sample under study and the atoms that occupy those planes. For a given wavelength λ and lattice plane spacing d, diffraction peaks will be observed when the X-ray beam is incident on a lattice plane at angles θ that satisfy the Bragg condition: nλ=2d sin θ, wherein n is the scattering order. The angle θ that satisfies the Bragg condition is known as the Bragg angle. Distortions in the lattice planes due to stress, solid solution, or other effects lead to observable changes in the XRD spectrum.
XRD has been used, inter alia, for measuring characteristics of crystalline layers produced on semiconductor wafers. For example, Bowen et al. describe a method for measuring germanium concentration in a SiGe structure using high-resolution XRD in “X-Ray metrology by Diffraction and Reflectivity,” Characterization and Metrology for ULSI Technology, 2000 International Conference (American Institute of Physics, 2001), which is incorporated herein by reference.